1. Field of the Invention
This invention relates to high strength, high performance, weathering plate steels with high yield strength, at least 70 ksi, preferably at least 75 ksi, and low yield strength-to-tensile strength ratio, and particularly, to thermomechanically controlled processing (TMCP) methods of manufacturing plates of such steels in long, e.g. about 90 to 120 foot, sections up to about 21/2 inches thick, without heat treatment such as quenching and tempering. Articles so made are especially useful for the fabrication of bridges and other constructional applications.
2. Prior Art
U.S. Pat. No. 2,586,042 discloses a low-alloy, high-yield strength (50 ksi) fabricable steel with superior resistance to atmospheric corrosion in thicknesses to about 1/2 inch [COR-TEN (later COR-TEN A); a registered trademark of U.S.Steel), ASTM A242], of medium carbon content (0.10-0.20 wt. %) and containing Mn, Ni, Cr, Mo (0.40-0.60 wt. %), V (0.03-0.10 wt. %), B, Si and Cu. A later modification (U.S. Pat. No. 2,858,206)--COR-TEN B (ASTM A588)--containing 0.12 wt. % C, with Mn, Si, Cu, Cr, Mo (0.15-0.45 wt. %), V (0.03-0.078 wt. %), Ti and B, was introduced to fill the need for a 50 ksi yield strength steel in plate thicknesses through about 4 inches. These two steels have been extensively employed in a variety of constructional applications such as railroad cars, bridges and exposed building framework elements.
Further improvements were made to these steels, including a relatively inexpensive steel with a minimum yield strength of 70 ksi, after quenching and tempering, in plate thicknesses to about 4 inches. "Mechanical Properties and Weldability of a 70 Ksi Minimum Yield Strength Steel for Bridge. Applications," (COR-TEN B-QT 70; ASTM A852 or A709 Grade 70W), U.S. Steel Technical Center Bulletin, Apr. 30, 1985. Such steels generally contained about 0.16-0.20 wt. % C, and such thick plates required a minimum preheat and interpass temperature of about 200-400.degree. F.
A recent publication by Nippon Steel Corporation, Development of High Performance Steels for Structures, K. Ichise et al., presents an overview of high performance steels and their manufacture, including use of the thermomechanical control processing (TMCP).
Despite the existence of such prior art steels, the need still exists for a steel having a minimum yield strength of 70 ksi with low yield/tensile ratio and producible in long, e.g. 90 foot, sections for, particularly, bridge and ship construction, and without the need for quenching and tempering (facilities for such heat treatments of such long sections do not exist; they are limited to about 50-55 foot lengths). Such long sections are of further advantage in reducing the number of splice welds of shorter sections and thus reduce costs and enhance appearance and performance of the fabricated structure.